by Abhijit S. Pandya; Ercan Sen CRC Press, CRC Press LLC
ISBN: 0849331390 Pub Date: 110198
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B. Signaling Mechanisms
We shall now discuss the signaling response for defining the procedures to be used when establishing connections over both, the interface between the end user and the ATM switch. In ATM switch networks, the user provides the information on the
virtual channels to the ATM nodes.
Any connection on the ATM network is referred to as a call and the process where the user informs the ATM network what connections, i.e., VPCs or VCCs, need to be set up is referred to as call control.
ATM signaling must be capable of doing a number of things, since ATM networks have a wide range of capabilities. For example, multi-connection calls can be set up between users and services like transferring voice; video and data
simultaneously may be required. During such a call connection it is important that these connections can be set up “on the fly”, when needed.
For video services, one cell sent into the network should be delivered to a number of end points. This would require a point to multi point type of connection. For more complex video and voice conferencing arrangements, a multi-point to multi-
point type of connection is required. Figure 2-6 shows the signaling sequence for a point-to-point connection. Figure 2-7 shows the multi-point signaling sequence.
Figure 2-6 Signaling sequence for point-to-point connection.
Figure 2-7 Signaling sequence for multi-point connections.
ATM signaling must be able to establish, maintain and release both the actual user virtual channel connections VCCs and the static virtual path connections VPCs in order to allow information flow across the network. It should be able to
negotiate the appropriate traffic and service characteristics of a connection.
In ATM, signaling is done by cells sent from the user to the network and back again using a logical signaling channel. Signaling is not performed over a channel used for traffic, so it is called out-of-band signaling. Current ITU standard ATM
VPIs are assigned at the service provision time on a per site basis, resulting in a “permanent” way for site connectivity. Such a VC establishment mechanism is called a Permanent Virtual Circuit PVC. However, the VCIs are set up through
“dynamic” connections between users at these sites by means of a signaling protocol. This kind of real-time signaling mechanism is called a Switched Virtual Circuit SVC.
These protocols are used by devices to set up, maintain, change and terminate connections across the ATM network. For example, the customer premise equipment CPE sends a message to the local network node requesting a connection to
Miami. If the destination Miami accepts the connection set up by the ATM network, it will send a message to the originator, such as, “connection OK, use VCI = 155.” VPI is predefined in this case as mentioned earlier.
The Permanent Virtual Connections PVCs are set up using an ATM Layer Management function on a node by node basis. In other words, segments of a PVC are established between each ATM node involved manually using the layer management
function. This type of connection requires coordination between each node and manual verification of the complete path end-to-end. However, due to the permanent nature of the connection, the manual set up overhead is acceptable.
On the other hand, the Switched Virtual Connections SVCs require a robust signaling protocol for call set up and release due to dynamic and on demand nature of the connections. The current ATM standard for ATM signaling is ITU-T Q2931
which is adapted by both ITU-T and the ATM Forum. The Q2931 is part of the UNI 3.1 specification. The Q.2931 is an adaptation of ITU-T Q931 ISDN Narrowband Signaling protocol with enhancements to support point-to-multi-point
connections. The latest version of the UNI specification UNI 4.0 adds additional capabilities such as negotiation of QoS parameters and VPIVCIs at the UNI interface.
Figure 2-8 ATM signaling.
Figure 2-8 illustrates the signaling mechanism across an ATM network.
Due to the fact that different addressing schemes are used today for private and public telecommunication domains, the ATM Forum recommendation on ATM addressing involves two sets of addressing schemes, one for private and one for
public ATM networks. An important element of this addressing concept is to maintain interoperability with existing networks which is one of the key objectives of ATM technology.
The ATM Forum recommends use of OSI’s Network Service Access Point NSAP address format for identification of end- users in private ATM networks and E.164 format for the public UNI interfaces in the public domain. The structure of the
E.164 ATM Addressing as recommended by the ATM Forum is shown in Figure 2-9. The upper 13 octets AFI, E.164,HO- DSP constitute the public domain part network prefix.
The network prefix part contains information about the granting authority and routing hierarchy. The eight-octet E.164 part contains the addressing information currently used in the worldwide telephone network. The lower seven-octet End-System
Identifier ESI field describes the identity of an end-user in the private network environment. For example, the ESI may represent the MAC address of a LAN workstation.
The Interim Local Management Interface ILMI bridges the public and private domains. When an end-station in a private LAN network wants to communicate to another end-user over the public ATM network, it uses the ILMI to register itself to
the ATM switch which serves as the public UNI interface gateway for the private network. The end-station sends its MAC address as ESI to the ATM switch and receives back a fully qualified E.164 ATM address from the ATM switch. The
address registration through ILMI is illustrated in Figure 2-10.
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by Abhijit S. Pandya; Ercan Sen CRC Press, CRC Press LLC
ISBN: 0849331390 Pub Date: 110198
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V. ATM Switching Concepts
